Localization of earth faults is a challenging task. There are many factors which deteriorate the accuracy of a calculated fault location estimate, such as fault resistance and load. Distribution networks are especially challenging as they have specific features which further complicate and challenge fault localization algorithms. These include e.g. non-homogeneity of lines, presence of laterals and load taps.
Impedance-based fault location algorithms have become industry standard in modern microprocessor-based protection relays. The reason for their popularity is their easy implementation as they utilize the same signals as the other functions. Their performance has proven to be satisfactory in localizing short-circuit faults, but they are often not capable of localizing low current earth faults, i.e. earth faults in high impedance earthed systems. This is due to the fact that an earth fault in high impedance earthed networks differs fundamentally from a short circuit fault.
Document “Earth fault distance computation with fundamental frequency signals based on measurements in substation supply bay”; Seppo Hänninen, Mafti Lehtonen; VTT Research Notes 2153; Espoo 2002, discloses an example of a method for fault localization of single phase earth faults in unearthed, Petersen coil compensated and low-resistance grounded networks. The disclosed method is based on measurements in a substation supply bay and it cannot therefore be optimally applied to feeder bays. Based on simulation results presented in the document, the performance of the algorithm is quite modest: with 2 MVA loading and 30 ohm fault resistance, the maximum error in 30 km line is −6.25 km i.e. −21%. With actual disturbance recordings one could expect even larger errors.
Prior art fault localization algorithms are typically based on an assumption that load is tapped to the end point of the electric line (e.g. a feeder), i.e. the fault is always assumed to be located in front of the load point. In real medium voltage feeders this assumption is rarely correct. In fact, due to voltage drop considerations, loads are typically located either at the beginning of the feeder or distributed more or less randomly over the entire feeder length. In such cases, the accuracy of prior art fault localization algorithms is impaired.